1. Field of the Invention
The present invention relates to a method of detecting the quantity of toner and, more particularly, to a method of detecting the quantity of toner in an image recording apparatus, capable of detecting the exhaustion and virtual exhaustion of toner, i.e., residual amount of toner in the storage on the basis of the output voltage of a toner quantity detector provided on the outer surface of the casing of a toner storage unit or a toner stirring unit.
2. Description of the Prior Art
An electrophotographic printer or a copying machine first forms an electrostatic latent image of the original document to be copied or printed on a photoconductive drum; thereafter prints or copies it by developing the electrostatic latent image formed by a toner transferring the toner image to a recording sheet and fixing the toner image on the recording sheet. FIG. 7 shows such an electrophotographic printer. The electrophotographic printer has a photoconductive drum 1, a charger 2 for charging the entire circumference of the photoconductive drum in a uniform potential, and a scanning unit 3 for longitudinally scanning the circumference of the photoconductive drum 1, (in the main scanning direction,) with a laser beam RB emitted by a laser diode, not shown, a developing unit 4 for developing the electrostatic latent image with toner, a transfer charger 5 for transferring the toner image to the recording sheet, a fixing unit 6 for fixing the toner image on the recording sheet, and a sheet transporting passage 7.
The scanning unit 3 comprises a polygonal rotating mirror 3a, an f.theta. lens 3b, a mirror 3c and a cylindrical lens 3d. During the scanning operation, the laser beam RB is turned on and off according to image data to form an electrostatic latent image on the photoconductive drum 1.
As shown in FIG. 8, the developing unit 4 comprises a toner stirring unit 4a for frictionally charging toner TN by stirring toner TN with a toner stirring member 4a-1, a toner feeder 4b and a toner quantity detector 4c for detecting the quantity of toner TN remaining in the developing unit 4. The toner TN is stirred by the toner stirrer 4a-1 and the frictionally charged toner TN is fed to a rotating magnet roller 4b-1 included in the toner feeder 4b. Surplus toner TN is scraped off with a doctor's blade 4b-2 to form a toner layer of toner TN in a predetermined thickness on the circumference of the magnet roller 4b-1 so that the toner layer touches the circumference of the photoconductive drum 1. A bias voltage is applied between the magnet roller 4b-1 and the photoconductive drum 1. The toner TN is transferred from the magnet roller 4b-1 to the electrostatic latent image formed on the circumference of the photoconductive drum 1 by the agency of the potential difference between the magnet roller 4b-1 and the photoconductive drum 1 to develop the electrostatic latent image in a toner image.
As shown in FIG. 9(a), the toner quantity detector 4c is placed fixedly in contact with a detector seat 4a-3, for mounting the sensor thereat formed on a frame 4a-2 of the developing unit 4 to detect the quantity of toner TN remaining in the developing unit 4. Since it is impossible to print a sharp image on a recording sheet if the quantity of toner TN in the developing unit 4 decreases excessively during operation, the quantity of toner TN is detected by the toner quantity detector 4c during operation and, if necessary, a warning is given to prompt the operator to replenish the developing unit 4 with toner or to replace the developing unit with another.
As shown in FIG. 9(b), the toner quantity detector 4c is of a differential transformer construction comprising a driving coil L1, a reference coil L2 and a detecting coil L3, which are mounted on a core. A high-frequency signal of 500 kHz is applied to the driving coil L1.
A developer, in general, is a mixture of a small amount of magnetic carrier particles and a magnetic toner. As toner is consumed, the magnetic resistance of the toner varies according to an upper level of the developer relative to the position of the toner quantity detector 4c, whether the upper level of which remains above a surface defined by the detector 4C or below the surface thereof or near the surface thereof and hence the output voltage of the detecting coil L3 varies with the variation of the level of the developer as shown in FIG. 9(c). That is, the output voltage of the toner quantity detector 4c remains constant on a high level while the quantity of toner is greater than a threshold quantity. When the quantity of toner decreases below the threshold quantity, the output voltage of the toner quantity detector 4c starts falling, and the output voltage of the toner quantity detector 4c settles at a low level after the toner has almost been exhausted.
A prewarning voltage E.sub.nep corresponding to a state where the toner is nearly exhausted or a warning voltage E.sub.emp corresponding to a state where the toner is virtually exhausted and is set or both the prewarning voltage E.sub.nep and the warning voltage E.sub.emp are set, the output voltage E.sub.s of the toner quantity detector 4c is compared with the prewarning voltage E.sub.nep and the warning voltage E.sub.emp to detect if the quantity of the toner has decreased below a predetermined quantity or if the toner has virtually been exhausted, and identifies the condition by an alarm.
The electrophotographic apparatus of such a construction uses a single-component developer comprising only a toner made of carbon particle containing a magnetic component therein, a two-component developer containing a nonmagnetic container and a magnetic carrier, or a two-component developer containing a small amount of magnetic carrier and a magnetic toner.
Incidentally, the output characteristic of the toner quantity detector is not constant due to a variation of the adjusting operation thereof and the sensitivity thereof. The variation of the adjustment is developed when an output of a magnetic resistance sensor portion of the detector is adjusted with a variable care.
Accordingly, the output characteristic of the toner quantity detector is represented by:
(1) a curve A (FIG. 10) when the variation of the sensitivity thereof is maximum and the variation of the the adjusting operation is maximum,
(2) a curve B (FIG. 10) when the variation of the sensitivity thereof is maximum and the variation of the adjusting operation is minimum,
(3) a curve C (FIG. 10) when the variation of the sensitivity thereof is minimum and the variation of the adjusting operation is maximum or
(4) a curve D (FIG. 10) when variation of the sensitivity from the standard is small and the variation of the adjusting operation is minimum.
If the output characteristic of the toner quantity detector varies according to the sensitivity and the condition of adjustment, the actual quantity of toner corresponding to the prewarning voltage E.sub.nep and that of the toner corresponding to the warning voltage E.sub.emp vary widely depending on the output characteristics represented by the output characteristic curves A, B, C and D. Accordingly, the number of copies that can be produced before the toner quantity detector provides an output voltage E.sub.s corresponding to the warning voltage E.sub.emp varies depending on the output characteristic of the toner quantity detector.
Therefore, all the possible output characteristics of the toner quantity detector must be taken into consideration in determining the prewarning voltage E.sub.nep and the warning voltage E.sub.emp ; that is, the prewarning voltage E.sub.nep must be set at a level below a voltage E1 defined by the output characteristic curve D, in which both variations of sensitivity and adjusting operation and minimum and the warning voltage E.sub.emp must be set at a level higher than a voltage E2 defined by the output characteristic curves A and C in which the adjusting operation thereof is maximum. If the prewarning voltage E.sub.nep and the warning voltage E.sub.emp are determined in such a manner, the difference between the prewarning voltage E.sub.nep and the warning voltage E.sub.emp is small, and hence the toner quantity detector will provide the warning voltage E.sub.emp in a very short time after providing the prewarning voltage E.sub.nep, so that only a very short time is available for preparing a supply toner after a prewarning has been given before a warning prompting the operator to replenish the developing unit with toner is given. If the operator is unable to prepare the supply toner before the warning is given, the operation of the electrophotographic apparatus must be interrupted.
Although toner quantity detectors have different output characteristics respectively due to errors in the toner quantity detector manufacturing processes and in the quality of the component parts, the toner quantity detectors are adjusted uniformly by a standardized adjusting method after mounting the toner quantity detectors respectively on electrophotographic apparatus before shipping the electrophotographic apparatus. Accordingly, it has been the present status of art that the output voltage E.sub.s of the toner quantity detector does not necessarily represent the accurate quantity of toner remaining in the developing unit.
As is obvious from FIG. 10, although the difference between the output voltages E.sub.s of the toner quantity detectors having different output characteristics near the warning voltage E.sub.emp is relatively small, the output voltages E.sub.s change sharply in greatly different modes from the upper output voltages above the prewarning voltage E.sub.nep to the lower output voltages near the warning voltage E.sub.emp, respectively. Thus, the mode of change of the output voltage E.sub.s from the upper output voltage to the lower output voltage is dependent on the inherent output characteristic of each toner quantity detector. Accordingly, the output characteristic of the toner quantity detector must be measured accurately to determine accurately the quantities of toner respectively corresponding to the prewarning voltage E.sub.nep and the warning voltage E.sub.emp of the toner quantity detector.